In the field of digital printing, a digital printer receives digital image data from a computer and places colorant on a receiver to reproduce the image. A digital printer can use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers.
It is a common goal in the field of digital printing to continually improve the quality of the output print, particularly when printing digital images of photographs. In recent years, advances in the technology related to digital printing have provided much opportunity for improving the quality of the output, particularly in the field of inkjet printing. An inkjet printer reproduces an image by ejecting small drops of ink from a printhead containing nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. A typical inkjet printer reproduces a color image by using a set of color inks, usually cyan, magenta, yellow, and black. Often, the dots produced by the inkjet printer are visible to the human eye, and result in an undesirable noise or “grainy” appearance to the reproduced image. Modern inkjet printers typically reproduce images using smaller ink drops than their predecessors, thus reducing the visibility of the ink dots and therefore improving the image quality. Another technique employed by some modern inkjet printers to reduce the grainy appearance of reproduced images is to use multiple inks of the same color, but different densities, such as a light cyan and a dark cyan ink. The light ink dots are less visible to the human eye than the dark ink dots, and regions of the image reproduced with the light ink will appear less grainy than if a dark ink had been used to reproduce the same region.
A number of different methods have been disclosed in the prior art for controlling the usage of the light and dark inks. For example, U.S. Pat. No. 4,635,078 discloses a inkjet printing system using a light ink and a dark ink. Dot patterns of the light ink are used to print light colors and dot patterns of the dark ink are used to print dark colors. A transition region is described for intermediate density levels where some light dots and some dark dots are printed.
U.S. Pat. No. 4,672,432 discloses another inkjet printing system using a light ink and a dark ink. Dot patterns are defined for a series of tone levels, where when there is more than one combination of colorants that can represent a given density, the combination having the largest number of drops is selected to minimize image graininess.
U.S. Pat. No. 4,727,436 discloses a system for printing an image with low and high density inks. The low density ink is used in a low density dot reproduction range, and the high density ink is used in a high density dot reproduction range. The transition between using the two inks is conducted where the picture density sharply changes.
Another common approach analogous to that disclosed in U.S. Pat. No. 5,142,374 is shown in FIG. 1. In this case, a single input printer code value I(x,y) is used to control all of the inks of a certain color at a given x-y location in the image. Consider the case of an inkjet printer having dark cyan (C) and light cyan (c) inks. With this approach, the dark cyan and light cyan inks would be controlled by a single cyan printer code value. A set of colorant control look-up tables 10 are provided to determine the amounts of the light and dark colorants as a function of the input printer code value I(x,y). The set of colorant control look-up tables 10 are sometimes referred to using terms such as “split LUTs” or “separation tables” or “ink manifolds.” In this example, the set of colorant control look-up tables 10 includes a light colorant control look-up table 11 and a dark colorant control look-up table 12, which are used to determine a light colorant control signal CL(x,y) and a dark colorant control signal CD(x,y), respectively. Next, halftoning operations are applied to determine the pattern of drops that should be printed given the colorant control signals. A halftone light colorant step 13 is used to process the light colorant control signal CL(x,y) to determine a light ink halftone signal HL(x,y). Similarly, a halftone dark colorant step 14 is used to process the dark colorant control signal CD(x,y) to determine a dark ink halftone signal HD(x,y). In many inkjet printing systems, the halftone signals will be binary where a “0” indicates that no ink drop should be printed at that x-y location and a “1” indicates that a single ink drop should be printed at that x-y location. Common halftoning methods that are used in many inkjet printers include the well-known error diffusion and blue-noise dither algorithms. The light ink halftone signal HL(x,y) is then used by a print light colorant step 15 to place the desired arrangement of light ink drops onto the paper. Likewise, the dark ink halftone signal HD(x,y) is then used by a print dark colorant step 16 to place the desired arrangement of dark ink drops onto the paper.
In some inkjet printing systems, more than two output levels can be produced. For example, it may be possible to print ink drops having a range of drop sizes, or to print multiple ink drops at a particular x-y position by using multiple passes of the print head over the paper. In such cases, the halftone steps can be adapted to output more than two output levels. Halftone algorithms that can produce more than two output levels are sometimes referred to as multi-level halftoning algorithms, or simply as multitoning algorithms.
Various configurations have been suggested for the form of the light colorant control look-up table 11 and the dark colorant control look-up table 12. For example, FIG. 2 illustrates a set of colorant control LUTs similar to those disclosed in FIG. 3 of U.S. Pat. No. 5,142,374. Two curves are given showing the relationship between the input printer code value and the corresponding colorant control signals for the two inks. A light colorant control look-up table 20 is used to control the light ink and a dark colorant control look-up table 21 is used to control the dark ink. It can be seen that as the input printer code value is increased, the colorant control signal for the light ink linearly increases until it reaches its maximum level. At this point the colorant control signal for the dark ink starts to increase while the colorant control signal for the light ink is decreased. At the maximum input printer code value, the colorant control signal for the dark ink is at its maximum level, while the colorant control signal for the light ink is at its minimum level. Similar configurations can be found in many prior art inkjet printing systems such as those disclosed in U.S. Pat. Nos. 5,729,259, 6,268,931 and 7,057,756.
A number of other configurations for the set of colorant control LUTs 10 have also been suggested in the prior art. For example, FIG. 3 shows an arrangement similar to that shown in FIG. 31 of U.S. Pat. No. 6,283,203. In this case, the dark colorant control look-up table 31 is similar to the dark colorant control look-up table 21 given in FIG. 2. However, a different form is used for the light colorant control look-up table 30 where the colorant control signal for the light ink stays at its maximum level rather than decreasing as the colorant control signal for the dark ink is increased.
U.S. Pat. No. 6,268,931 discloses a configuration where the low density ink amount is limited to a value which for each color would result in no more than fifty percent of the dots in the final halftone pattern being low density ink dots. U.S. Pat. No. 7,057,756 discloses a configuration where a higher level of light ink is used (e.g., 200%) so that the use of dark ink can be delayed to a higher gradation level. U.S. Pat. No. 6,312,101 discloses a method to create a set of colorant control LUTs by minimizing a cost function. This permits the system designer to trade off various attributes such as granularity, total ink volume and relative ink usage.
While most prior art configurations utilizing light inks describe systems having two inks of the same color (e.g., light and dark cyan), the basic approach can easily be generalized to systems having more than two inks of the same color. For example, U.S. Pat. No. 5,142,374 describes a configuration having a light ink, a medium ink and a dark ink. Increasing the number of different ink densities for a given color has the advantage that lower granularity levels can be obtained at the expense of additional system cost and complexity. Therefore, such configurations are typically limited to applications where image quality is a high priority. FIG. 4 shows a generalization of the prior art configuration given in FIG. 1 for the case where 5 inks of substantially the same color but different densities are used. In this configuration, a set of colorant control look-up tables 40 are provided to determine the amounts of the 5 colorants as a function of the input printer code value I(x,y). The set of colorant control look-up tables 40 includes colorant control look-up tables 42 for each of the individual colorants, which are addressed by the input printer code value I(x,y). The output of the colorant control look-up tables 42 are the colorant control signals CN(xy) for each of the colorants, where N indicates the colorant number. Halftoning operations 44 are applied to the colorant control signals CN(xy) to determine halftone signals HN(x,y) for each of the colorants. Print colorant steps 46 are then used to print the halftone signals HN(x,y) for each of the colorants.
FIG. 5 shows a set of colorant control look-up tables that can be used in accordance with the five-ink configuration shown in FIG. 4. It can be seen that this is a generalization of the two-colorant control look-up tables shown in FIG. 3. This arrangement limits the total ink usage to a maximum of 200% of the maximum ink amount for any individual ink. A first colorant control look-up table 51 is used to control the lightest colorant. As with FIG. 3, the colorant control signal for the lightest colorant increases linearly to its maximum level and stays there while the second colorant is linearly increased. However, in this case, once a second colorant control look-up table 52 reaches its maximum level, the first colorant control look-up table 51 is decreased as a third colorant control look-up table 53 is linearly increased. This pattern is repeated for a fourth colorant control look-up table 54 and a fifth colorant control look-up table 55.
U.S. Pat. Nos. 5,142,374 and 6,268,931 have suggested that for a two-ink configuration, the density produced by full coverage of the light ink should be one half of the density produced by full coverage of the dark ink. For configurations with more than two inks of the same color, this principal can be generalized such that the colorant concentrations for the set of inks are selected so that the optical densities of the individual inks are equally spaced.
For any given ink, it is generally found that there is a non-linear relationship between the resulting density and the colorant control signal for that ink. The exact form of these nonlinearities will depend on a number of different factors such as the inkjet printing configuration (which includes parameters such as drop size(s), maximum number of ink drops, number of printing passes), the type of halftoning, and the characteristics of the ink and the receiver. A representative set of tone scale curves are shown in FIG. 6 for a five-ink system where the optical densities for full coverage of the individual inks are equally spaced. A first tone scale curve 61 shows the density vs. colorant control signal relationship for the lightest ink. Similarly, tone scale curves 62 through 65 show the density vs. colorant control signal relationships for the other four inks.
Several problems that can arise in the prior art arrangements are illustrated in FIG. 7. This figure shows an example composite tone scale function 70 that can result from the conventional five-ink configuration like that shown in FIG. 4 using the conventional set of colorant control look-up tables shown in FIG. 5. The composite tone scale function 70 in FIG. 7 shows the relationship between the input printer code value and the resulting “lightness” of the printed image. (The composite tone scale function 70 was determined using a model of an inkjet system using a set of five gray inks that were equally spaced in optical density.) The lightness in this plot is the L* value of the well-known CIELAB color space, which is intended to be an approximate representation of the response of the human visual system.
The solid circles plotted on the composite tone scale function 70 correspond to a series of node points where each of the inks are at either their maximum or minimum colorant amounts. These points correspond to the input printer code values in FIG. 5 where there are sharp corners in the colorant control LUTs. The first node point at an input printer code value of 0 corresponds to no ink being applied; the second node point at an input printer code value of 52 corresponds to the maximum amount of the lightest ink and none of the other inks; and the third node point at an input printer code value of 102 corresponds to maximum amounts of the first and second inks and none of the other inks. For the other three node points, two of the inks are at their maximum levels, and the other three inks are at their minimum levels.
One undesirable characteristic of the composite tone scale function 70 is that it is highly nonlinear, and consequently most of the lightness change happens in the first half of the input printer code value range. This largely results from the fact that there is not a simple additive relationship when multiple inks are combined, together with the fact that there is a nonlinear relationship between optical density and L*. This nonlinear shape can complicate the design of the calibration and color management operations in an inkjet printing system since conventional approaches typically don't work well when the output response does not vary significantly over some range of input printer code values.
A more serious problem with the composite tone scale function 70 is the fact that it is not smooth, having a series of serrations between the node points. These serrations are prone to produce contouring artifacts and create further complications for the design of the calibration and color management operations. This problem is particularly troublesome if the serrations grow large enough so that the composite tone scale function becomes non-monotonic.
U.S. Pat. No. 7,262,882 recognizes the problems associated with a nonlinear tone scale response, and suggests that a “linearly-changing” density curve can be achieved by modifying a conventional set of colorant control look-up tables like those shown in FIG. 2. This is accomplished by re-sampling the input printer code value axis at a series of points corresponding to equally-spaced density values. However, this approach has the serious limitation that it does not work for cases where the composite tone scale function has flat spots or non-monotonicities.